Migrate SystemTest tests to ApplicationSystemTestCase

[colby-swandale]

Currently, the rubygems.org test suite has 2 different kinds of System tests, one that is using the rails ApplicationSystemTestCase and the other, a cut-down version using rack_test as the driver called SystemTest. It's hard to tell when we should be using one or the other, and the benefit of keeping it does not seem high either.

In an effort to simplify our test suit, i want to remove SystemTest and migrate everything to ApplicationSystemTestCase.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.18%. Comparing base (23fa363) to head (a151945).
Report is 15 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5783   +/-   ##
=======================================
  Coverage   97.18%   97.18%           
=======================================
  Files         469      469           
  Lines        9676     9676           
=======================================
  Hits         9404     9404           
  Misses        272      272           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.